Steps in short

In .spacemacs:

(defun dotspacemacs/user-config ()
  "Configuration function for user code.
This function is called at the very end of Spacemacs initialization after
layers configuration.
This is the place where most of your configurations should be done. Unless it is
explicitly specified that a variable should be set before a package is loaded,
you should place your code here."
;; some other setq ...

  (setq org-agenda-skip-scheduled-if-done t)
  (setq org-agenda-skip-deadline-if-done t))

Story behind

After install this magnificent calendar view to spacemacs I firgured, that my calendar is littered with DONE items, at least in the month view. calfw allows you filter entries with cfw:org-agenda-schedule-args, but they concern only the types of entries (you can filter out all deadlines, scheduled or sexps), but does not allow one to filter out by DONE or TODO.

Luckily, a quick search with SPC h d v found that agenda can actually skip done items.

God bless developers of org-mode.